CN112886611B - Subsynchronous oscillation suppression method for direct-drive fan grid-connected system - Google Patents

Subsynchronous oscillation suppression method for direct-drive fan grid-connected system Download PDF

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CN112886611B
CN112886611B CN202110073820.3A CN202110073820A CN112886611B CN 112886611 B CN112886611 B CN 112886611B CN 202110073820 A CN202110073820 A CN 202110073820A CN 112886611 B CN112886611 B CN 112886611B
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direct
grid
drive fan
subsynchronous oscillation
connected system
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CN112886611A (en
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韩平平
王希
陆中来
崔晓丹
吴家龙
王欢
汪宗强
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Hefei University of Technology
NARI Group Corp
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241The oscillation concerning frequency
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/04Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
    • H02J3/06Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20The dispersed energy generation being of renewable origin
    • H02J2300/28The renewable source being wind energy
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/10Flexible AC transmission systems [FACTS]

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  • Control Of Eletrric Generators (AREA)
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Abstract

The invention discloses a subsynchronous oscillation suppression method of a direct-drive fan grid-connected system, which comprises the following steps of: 1. establishing a full-system mathematical model of a direct-drive fan power generation system merged into a weak alternating current power grid, and establishing a small-signal model after linearization; 2. performing characteristic root calculation based on a small signal model, judging a subsynchronous oscillation mode, calculating participation factors of the subsynchronous oscillation mode, obtaining the participation degree of each state variable to the subsynchronous oscillation mode, and determining components influencing subsynchronous oscillation; 3. a simulation model of a direct-drive fan grid-connected system is built on the basis of Matlab/Simulink simulation software, and a phase-shifting transformer is added to a PCC point. The invention can isolate the interaction between the network side converter and the weak alternating current power grid from the source, thereby inhibiting subsynchronous oscillation.

Description

Subsynchronous oscillation suppression method for direct-drive fan grid-connected system
Technical Field
The invention belongs to the technical field of direct-drive fans, and particularly relates to a subsynchronous oscillation suppression method for a direct-drive fan grid-connected system.
Background
With the popularization of new energy power generation in China, the installed capacity of wind power generation is increased year by year, wherein the direct-drive fan has the advantages of convenience in maintenance, good reliability, high efficiency and the like due to the fact that the direct-drive fan is free of a gear box and an excitation control system, and the direct-drive fan is widely applied to wind power generation.
However, as various new energy power generation grid connection and power electronic components cover all links of the power system, the stability of the power grid is gradually weakened, and the risk of subsynchronous oscillation caused by the fact that the direct-drive fan is not connected to the grid through the series compensation device is increased. In 2015, when a certain direct-drive wind power plant in Xinjiang in China supplies power to a nearby series compensation-free alternating current power grid, a continuous subsynchronous oscillation phenomenon is caused, so that a steam turbine set which is hundreds of kilometers away along the line generates serious generator tripping accidents due to protection actions, and the stability of a regional power grid is influenced. Although a great deal of research has been conducted on subsynchronous torsional vibration caused by a large steam turbine set and subsynchronous control interaction phenomena caused by the fact that a doubly-fed induction fan is connected to a high-series-compensation-degree power grid, the subsynchronous oscillation phenomena caused by a direct-driven fan are observed for the first time in 2015, and the mechanism of the engineering accident is not clearly elucidated in academic circles.
Aiming at the problem of the sub-synchronous oscillation suppression of the wind power grid connection, two measures are commonly used, including: firstly, adjusting the control coefficient of a converter according to the influence trend of the control coefficient of the converter on subsynchronous oscillation, which can cause that the converter of the fan cannot reach the optimal control target; and secondly, a damping control link or a virtual resistance link is introduced, but the damping control link or the virtual resistance link is limited by controller hardware in the engineering practice, and an additional control loop is difficult to add in the built operating wind power plant. Meanwhile, the two measures are designed aiming at a single subsynchronous oscillation mode, and in fact, in 2015, a plurality of subsynchronous oscillation modes are monitored in the subsynchronous oscillation accident of the Xinjiang direct-drive fan, so that the engineering applicability of the current suppression measure is poor.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, and provides a subsynchronous oscillation suppression method of a direct-drive fan grid-connected system, so that interaction between a grid-side converter and a weak alternating current power grid can be isolated from the source, and subsynchronous oscillation is suppressed.
The technical scheme adopted by the invention for solving the technical problem is as follows:
the invention discloses a subsynchronous oscillation suppression method of a direct-drive fan grid-connected system, which is characterized by comprising the following steps of:
step 1, establishing a dynamic mathematical model of each component in a direct-drive fan grid-connected system, wherein the direct-drive fan grid-connected system is formed by merging a direct-drive fan power generation system into a weak alternating current power grid, and the direct-drive fan power generation system is formed by a wind turbine, a shafting, a permanent magnet synchronous generator, a machine side converter, a grid side converter, a PLL (phase locked loop) and a direct current capacitor;
step 2, according to the dynamic mathematical model of each component, obtaining a full-system mathematical model of the direct-drive fan power generation system merged into the weak alternating current power grid, and carrying out linearization processing on the full-system mathematical model, so as to establish a small-signal model of the direct-drive fan grid-connected system;
step 3, performing characteristic root calculation on the state matrix of the small signal model to obtain a left characteristic vector, a right characteristic vector and a characteristic value; selecting a subsynchronous oscillation mode of the direct-drive fan grid-connected system from the characteristic values, and judging the stability of the subsynchronous oscillation mode;
step 4, utilizing the left and right eigenvectors to perform normalized participation factor calculation on the subsynchronous oscillation mode to obtain the participation degree of each state variable in a state matrix to the subsynchronous oscillation mode, so as to judge that the subsynchronous oscillation of the direct-drive fan grid-connected system is generated by the interaction of a grid-side converter and a weak alternating current power grid;
step 5, building a simulation model of a direct-drive fan grid-connected system in Matlab/Simulink simulation software, and using the simulation model to reproduce the subsynchronous oscillation phenomenon and obtain a response curve of the network side A-phase voltage changing along with time and specific subsynchronous oscillation frequency distribution of the response curve;
step 5.1, the simulation model of the direct-drive fan grid-connected system comprises the following steps: the system comprises a pitch angle control system, a wind turbine, a permanent magnet synchronous generator, a crowbar protection circuit, a machine side converter and a control system thereof, a network side converter and a control system thereof, a direct current capacitor, a filter, a PLL (phase locked loop), a step-up transformer, a power transmission line and a three-phase infinite power supply;
step 5.2, increasing the equivalent reactance of the simulation model of the direct-drive fan grid-connected system by increasing the length of the power transmission line so as to construct a weak alternating current power grid environment;
step 5.3, inputting voltage U into the direct-drive fan grid-connected system model z Wind speed v w Total impedance R + jX of power transmission line and reference frequency f N Running simulation to obtain a response curve of the voltage of the A phase on the network side changing along with time, and carrying out FFT analysis on the response curve to obtain the distribution of frequency f, wherein if the frequency f is more than 2.5Hz and less than 50Hz and the waveform of the voltage of the A phase on the network side is oscillated and dispersed, the subsynchronous oscillation phenomenon is represented;
and 6, adding a phase-shifting transformer to the PCC point to replace the original booster transformer so as to inhibit subsynchronous oscillation.
Compared with the prior art, the invention has the beneficial effects that:
1. the invention obtains the influence factors of the subsynchronous oscillation mode by using a small signal analysis method, has clear internal mechanism and is convenient for directly finding out key components for causing the subsynchronous oscillation of the direct-drive fan in the weak power grid from a physical layer.
2. The method utilizes the small signal model to judge that the subsynchronous oscillation phenomenon of the direct-drive fan is generated by the interaction of the grid-side converter and the weak alternating-current power grid; therefore, a phase-shifting transformer is added to the PCC point, so that the interaction between the grid-side converter and the weak alternating current power grid can be blocked from the source, the subsynchronous oscillation is inhibited, the control structure or the control parameters of the grid-side converter do not need to be changed, and the optimal control target is realized.
3. The current suppression measures are generally designed aiming at a single subsynchronous oscillation mode generated by a single wind turbine or a single equivalent wind field, however, in practice, the difference between units in a large-scale wind power plant is large, complex interaction exists among the units, and a plurality of subsynchronous oscillation modes generally exist when a subsynchronous oscillation phenomenon occurs.
4. The phase-shifting transformer can also be used for controlling the power flow distribution of a power system, improves the running stability of a power grid, ensures the reliability of a large-scale wind power plant through a high-voltage direct-current remote output project, and has better economy.
Drawings
FIG. 1 is a block diagram of a machine side converter control system according to the present invention;
FIG. 2 is a block diagram of a grid-side converter control system according to the present invention;
FIG. 3 is a topological structure diagram of a direct-drive fan grid-connected system in the invention;
FIG. 4 is a graph of the normalized participation factor calculation for the sub-synchronous oscillation mode in an example of the present invention;
FIG. 5a is a simulation of the A-phase voltage with varying short circuit ratios in an example of the present invention;
FIG. 5b is a graph of FFT analysis of the A-phase voltage with varying short circuit ratios in accordance with an embodiment of the present invention;
FIG. 6 is a simulation of the A-phase voltage with the addition of a phase shifting transformer in accordance with an embodiment of the present invention.
Detailed Description
The technical solution of the present invention will be further described in detail with reference to the following embodiments and accompanying drawings
In this embodiment, a method for suppressing subsynchronous oscillation of a direct-drive fan grid-connected system is performed according to the following steps:
step 1, establishing a dynamic model of each component in a direct-drive fan grid-connected system;
the direct-drive fan grid-connected system is formed by merging a direct-drive fan power generation system into a weak alternating current power grid; the direct-drive fan power generation system mainly comprises a wind turbine, a shafting, a permanent magnet synchronous generator, a back-to-back converter, a PLL (phase locked loop) and a direct current capacitor;
step 1.1, establishing a wind turbine model by using the formula (1):
Figure GDA0003634957930000031
in the formula (1), R is the radius of a wind turbine blade; c p A wind energy utilization factor; ρ is the air density; v. of w Is the wind speed;ω t the rotating speed of the wind turbine; t is M Is the mechanical torque of the wind turbine.
Step 1.2, establishing a single mass block shafting model by using the formula (2):
Figure GDA0003634957930000032
in the formula (2), J is the rotational inertia of the permanent magnet synchronous generator.
Step 1.3, establishing a permanent magnet synchronous generator model by using the formula (3):
T e =1.5N p i sq ψ f (3)
in the formula (3), T e The torque is the electromagnetic torque of the permanent magnet synchronous generator; psi f Is a magnetic flux; n is a radical of p Is the number of pole pairs of the generator.
Step 1.4, establishing a machine side converter model by using the formula (4), wherein the current flow direction adopts a motor convention, and a machine side converter control system block diagram is shown in figure 1;
Figure GDA0003634957930000041
in the formula (4), i sd 、i sq D-axis and q-axis components of the stator current, respectively; u. of sd 、u sq D and q axis components of the stator voltage, respectively; r s 、L s Respectively a stator resistor and an inductor; omega r Is the generator speed; k p1 、K i1 D-axis current proportion and integral gain respectively; k p2 、K i2 Q-axis current proportion and integral gain respectively; k p3 、K i3 The rotating speed outer ring proportion and integral gain are respectively.
Step 1.5, establishing a PLL model by using the formula (5):
Figure GDA0003634957930000042
in the formula (5), u gq Q is the PCC point voltageAn axial component; omega PLL The grid angular velocity measured for the phase-locked loop; omega g Is a reference angular velocity; k p_PLL 、K i_PLL The distribution is PLL proportional, integral gain.
Step 1.6, establishing a network side converter model by using the formula (6), wherein a block diagram of a network side converter control system is shown in fig. 2;
Figure GDA0003634957930000051
in the formula (6), i gd 、i gq D-axis components and q-axis components of the outlet current of the grid-side converter are respectively; u. of id 、u iq D-axis components and q-axis components of the grid-side converter outlet voltage are respectively; u. of dc The voltage of a direct current side capacitor of the grid side converter is obtained; l is g Is a filter inductor; u. of gd Is the d-axis component of the PCC point voltage; k p4 、K i4 Respectively are direct current voltage proportion and integral gain; k p5 、K i5 D-axis current proportion and integral gain respectively; k p6 、K i6 The q-axis current proportion and the integral gain are respectively.
Step 1.7, establishing a direct current capacitor model by using the formula (7):
Figure GDA0003634957930000052
in the formula (7), C dc Is a DC side capacitor of the grid side converter.
Step 1.8, establishing a power transmission line model by using the formula (8):
Figure GDA0003634957930000053
in the formula (8), i ld 、i lq D-axis components and q-axis components of the current of the power transmission line are respectively; u. of zd 、u zq D and q axis components of the grid voltage respectively; l is 1 、R 1 、C 1 Respectively arranging an inductor, a resistor and a capacitor of the power transmission line; k is step-up transformerAnd (4) transformation ratio.
And 2, simultaneously deducing a full-system mathematical model of the direct-drive fan merged into the weak alternating-current power grid according to the model of each element in the step 1, and establishing a small-signal model of the system through linearization.
Step 2.1, establishing a full-system mathematical model by using the formula (9), wherein the full-system mathematical model consists of 18 differential equations and 7 algebraic equations;
Figure GDA0003634957930000061
step 2.2, selecting a balance point v w Calculating the initial value of each variable at 11m/s, substituting the initial value into an equation (9), and linearizing the initial value to establish a small signal model of the system, as shown in equation (10):
Figure GDA0003634957930000062
in the formula (10), Δ x and Δ u are respectively a state variable and an input variable after linearization; the matrix H, F is a state matrix and an algebraic matrix of the small signal model respectively; there are 18 state variables and 10 input variables in the small-signal model, where the state variable x ═ ω t ,x 1 ,x 2 ,x 3 ,i sd ,i sq ,x PLLPLL ,x 4 ,x 5 ,x 6 ,i gd ,i gq ,u dc ,u gd ,u gq ,i ld ,i lq ]。
And 3, performing characteristic root calculation on the state matrix H according to the small signal model established in the step 2 to obtain an oscillation mode of the direct-drive fan merged into the weak alternating-current power grid, wherein the formula is shown as (11):
λ 1,2 =σ±j2πf (11)
step 3.1, if f is more than 2.5Hz and less than 50Hz, the oscillation mode is a subsynchronous oscillation mode;
step 3.2, if the sigma is less than 0, no oscillation or oscillation convergence occurs under the frequency;
3.3, if the sigma is larger than 0, the oscillation mode under the frequency is unstable;
according to the steps, the subsynchronous oscillation mode of the system can be selected, and the stability of the subsynchronous oscillation mode is judged.
And 4, carrying out normalized participation factor calculation on the subsynchronous oscillation mode to obtain the participation degree of each state variable to the subsynchronous oscillation mode, and judging components influencing the subsynchronous oscillation mode in the whole system from a physical layer, namely the interaction between the network side converter and the weak alternating current power grid.
And 5, building a direct-drive fan grid-connected system model in Matlab/Simulink simulation software as shown in FIG. 3.
Step 5.1, establishing a direct-drive fan power generation system model, which mainly comprises a pitch angle control system, a wind turbine, a permanent magnet synchronous generator, a crowbar protection circuit, a machine side converter and a control system thereof, a network side converter and a control system thereof, a direct current coupling capacitor, a filter, a PLL and a step-up transformer;
step 5.2, establishing a weak alternating current power grid model, wherein the strength of the power grid is generally represented by a Short Circuit Ratio (SCR), the SCR is represented by a formula (12), and when the SCR is more than 2 and less than 10, the power grid is a weak power grid; when SCR is less than 2, the grid is very weak.
Figure GDA0003634957930000071
In the formula (12), P SC Short circuit capacity of the system; p N Rated capacity for accessing power grid equipment; x is the number of pu Is the per unit value of the equivalent reactance of the system.
According to equation (12), by increasing the total reactance of the transmission line, the SCR can be lowered to construct a weak ac grid environment.
Step 5.3, running simulation, and carrying out voltage U on A phase at the network side a And carrying out FFT analysis to obtain specific subsynchronous oscillation frequency distribution.
And 6, adding a phase-shifting transformer (PST) to the PCC point to replace the original step-up transformer, namely, putting the phase-shifting transformer in the graph 3 into the PCC point, and cutting out the original step-up transformer, so that on one hand, the phase deviation of primary side voltage and secondary side voltage is realized, the harmonic of subsynchronous frequency components in the output voltage of the grid-side converter of the direct-drive fan is eliminated, on the other hand, the grid-side converter is electrically isolated from a weak alternating current power grid, the interaction between the grid-side converter and the weak alternating current power grid is blocked, and the purpose of inhibiting subsynchronous oscillation is achieved.
Example (b):
taking a large power grid with a 30kv voltage level connected to a single fan as an example:
TABLE 1 direct-drive blower grid-connected system parameters
Figure GDA0003634957930000072
1. Small signal modeling and analysis are carried out according to the parameters in the table 1, SCR is set to be 2, and 1 group of unstable subsynchronous oscillation modes lambda is obtained 1,2 The subsynchronous oscillation mode shows negative damping at 0.825 ± j222.739, the corresponding subsynchronous oscillation frequency is 35.45Hz, and the calculation result of the normalized participation factor is shown in fig. 4. From FIG. 4, it can be seen that the subsynchronous oscillation mode is given by parameter i gd 、i gq 、u gd 、u gq The influence degree of the direct-drive wind turbine is large, so that the reason that the direct-drive wind turbine subsynchronous oscillation is generated due to the interaction of the grid-side converter and the weak alternating current power grid can be determined.
2. A direct-drive fan grid-connected model is established in Simulink simulation software according to the parameters in the table 1, the system short-circuit ratio is set to be 2.8, the total simulation time is 6s, and when the simulation runs to the 2 nd s, the short-circuit ratio is reduced to be 2. As shown in fig. 5a, the 2s back-grid side a-phase voltage waveform oscillates divergently; meanwhile, FFT analysis is carried out on the waveform, as shown in figure 5b, subsynchronous component 15Hz and supersynchronous component 85Hz are contained, and subsynchronous oscillation occurs in a 2s rear direct-drive fan grid-connected system.
3. The total simulation time is 6s, when the simulation system runs to the 2 nd s, a phase-shifting transformer is additionally arranged on the PCC point, and the phase-shifting angle of the phase-shifting transformer is set to be +5 degrees. As shown in fig. 6, unstable subsynchronous oscillation occurs before 2s, a phase voltage waveform oscillation of the a phase diverges, and the system recovers to be stable after 2s, thereby achieving the goal of suppressing the subsynchronous oscillation.

Claims (1)

1. A subsynchronous oscillation suppression method of a direct-drive fan grid-connected system is characterized by comprising the following steps:
step 1, establishing a dynamic mathematical model of each component in a direct-drive fan grid-connected system, wherein the direct-drive fan grid-connected system is formed by merging a direct-drive fan power generation system into a weak alternating current power grid, and the direct-drive fan power generation system is formed by a wind turbine, a shafting, a permanent magnet synchronous generator, a machine side converter, a grid side converter, a PLL (phase locked loop) and a direct current capacitor;
step 2, according to the dynamic mathematical model of each component, obtaining a full-system mathematical model of the direct-drive fan power generation system merged into the weak alternating current power grid, and carrying out linearization processing on the full-system mathematical model, so as to establish a small-signal model of the direct-drive fan grid-connected system;
step 3, performing characteristic root calculation on the state matrix of the small signal model to obtain a left characteristic vector, a right characteristic vector and a characteristic value; selecting a subsynchronous oscillation mode of the direct-drive fan grid-connected system from the characteristic values, and judging the stability of the subsynchronous oscillation mode;
step 4, utilizing the left and right eigenvectors to perform normalized participation factor calculation on the subsynchronous oscillation mode to obtain the participation degree of each state variable in a state matrix to the subsynchronous oscillation mode, so as to judge that the subsynchronous oscillation of the direct-drive fan grid-connected system is generated by the interaction of a grid-side converter and a weak alternating current power grid;
step 5, building a simulation model of a direct-drive fan grid-connected system in Matlab/Simulink simulation software, and using the simulation model to reproduce the subsynchronous oscillation phenomenon and obtain a response curve of the network side A-phase voltage changing along with time and specific subsynchronous oscillation frequency distribution of the response curve;
step 5.1, the simulation model of the direct-drive fan grid-connected system comprises the following steps: the system comprises a pitch angle control system, a wind turbine, a permanent magnet synchronous generator, a crowbar protection circuit, a machine side converter and a control system thereof, a network side converter and a control system thereof, a direct current capacitor, a filter, a PLL (phase locked loop), a step-up transformer, a power transmission line and a three-phase infinite power supply;
step 5.2, increasing the equivalent reactance of the simulation model of the direct-drive fan grid-connected system by increasing the length of the power transmission line so as to construct a weak alternating current power grid environment;
step 5.3, inputting voltage U into the direct-drive fan grid-connected system model z Wind speed v w Total impedance R + jX of power transmission line and reference frequency f N Running simulation to obtain a response curve of the voltage of the A phase on the network side changing along with time, and carrying out FFT analysis on the response curve to obtain the distribution of frequency f, wherein if the frequency f is more than 2.5Hz and less than 50Hz and the waveform of the voltage of the A phase on the network side is oscillated and dispersed, the subsynchronous oscillation phenomenon is represented;
and 6, adding a phase-shifting transformer to the PCC point to replace the original booster transformer so as to inhibit subsynchronous oscillation.
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